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Re: [TCML] Frequencies
Jim,
Thanks. Once again, you are quite correct and I'm really glad you added your
comments. I hadn't thought of the reasons you give for my 3055 failures and
they make perfect sense. No wonder I fried so many 3055's. I was barking up
the wrong tree.
I'm not sure overvoltage was the problem and I had been using the 3055HV's
rated to 100Vceo for a little extra leeway in this regard and they are also
rated Vbc 100V but I am sure I have been guilty of under drving the base. I
suppose powering the 555 with a nine volt battery limits the amount of base
current somewhat? :-) Also, the 3055HV is advertised as 20hfe mimimum and
100 maximum but I certainly don't believe they ever produce anything more
than 20 and wonder if they even do this. One problem with the 3055HV and a
reason I use a huge heatsink is that the 3055HV is only rated 90 watts, not
the 115 watts of the regular 3055 but I usually use lots of heatsinking with
most of my circuits and add fans a lot, too. I feel like there is no such
thing as too much heatsink.
I was powering the ignition coil portion of my circuit, which is essentially
the same as the one Rhys is using, with 24V via two, 12V, 12AH gel cell
batteries in series. The 555 circuit was powered completely separately,
except for common ground, initially with a 9 volt transistor radio battery
(PP3) and then an 8.2 volt NiCd pack. I did not provide any sort of
protection to or around the 3055 and I wonder if using a diode, as shown in
Rhys's circuit schematic, would help, especially driving an inductive load
such as this? I hadn't thought about any of this for a long time. Any
thoughts or further comments, please?
Paul
Think Positive
----- Original Message -----
From: jimlux
To: Tesla Coil Mailing List
Sent: Monday, October 12, 2009 09:57
Subject: Re: [TCML] Frequencies
Paul Brodie wrote:
Rhys,
That is a very old and very well tested ciruit that works fine. The diode
won't hurt but is not necessary at all. I have used this circuit and
modifications of it quite a bit. I would like to pass on to you that the
3055 transistors are way over rated and will not stand this for very long
before they "blow" and let all of their smoke out. They claim to be rated
at 15 amps but I have yet to see one take much more than 5 amps for long,
even with a massive heat sink.
Funny, I've got 3055s in power supply duty doing 10A day in and day out.
I suspect you're killing them with overvoltage. I've killed many a
3055 in PWM switcher duty, before I got smart and chose a different
transistor. The 3055 has a BVce of 60V, and, more to the point a pretty
low reverse voltage limit on the base emitter junction.
The other thing to watch out for on the 3055 is that the hFE (large
signal current gain) is really, really low (like 10 or less).. so you
need a LOT of base drive, or it drops out of saturation, and the
dissipation goes up, or worse, with an inductive load, the decrease in
current causes a increase in Vce, killing it with overvoltage.
As you note, it needs a decent heatsink, and probably forced air
cooling, if you'rre going to get close to the 150W rating.
By the way, be sure to use a
large heat sink for the 3055 transistor to have it last more than a few
seconds.Also, the circuit works much better with a separate power supply
across the ignition coil from the one powering the 555 IC. This way, you
are able to use much higher voltages across the ignition coil without the
15 volt limitation of the 555.
You will be much better served by a simple modification and use a MosFet
instead of the 3055. There are many MosFet's available with much higher
voltage and amp ratings than the 3055 and they are just as inexpensive.
Indeed.. Or a IGBT or a decent darlington. There are also power
transistors MADE for driving ignition coils that have a 400V max Vceo,
and a good breakdown characteristic (almost like they have a 400V zener
built in). I can't recall the number off hand.. If you hunt down the
data sheet for the old motorola electronic ignition IC, there's a wealth
of useful information.
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